REPRODUCTION. 677 



offspring. The fact cannot therefore be accepted as affording 

 conclusive evidence. 



But to return. The second point to be considered is the 

 nature of the variation manifested by the variable organism. 

 Darwin points out that, besides the indefinite effect already 

 mentioned, a change in the conditions of life has what he 

 terms a definite effect upon the organism in determining the 

 nature of the variation. The effect, he says, may be con- 

 sidered definite when all or nearly all the offspring of in- 

 dividuals exposed to certain conditions during several gene- 

 rations vary in the same manner. At the same time he 

 points out that it is very difficult to decide how far changed 

 conditions have acted in a definite manner; and the difficulty 

 is increased with regard to variations which are advantageous, 

 to Adaptations that is, inasmuch as in this case we cannot tell 

 how much to attribute to the definite action of the changed 

 conditions, on the one hand, and to the accumulative action of 

 natural selection on the other. Yet, he concludes, there is 

 reason to believe that in the course of time the effect of 

 the definite action of the changed conditions has been greater 

 than can be proved by clear evidence. Darwin considers, 

 then, that changes in external conditions are ever acting 

 upon living organisms and that consequently new varieties 

 are ever being produced. 



An altogether different view of the origin of varieties 

 is held by those who, like Weismann, deny that characters 

 acquired by individuals under the influence of changed ex- 

 ternal conditions can be transmitted in sexual reproduction. 

 It is true that in many cases the modifications thus produced, 

 especially those due to a change in climate or nutrition, 

 are individual and transitory. For instance, a wild plant 

 which naturally grows in poor soil will, when transplanted 

 into rich soil, assume a very different habit; and the seeds 

 of the modified plant, if sown in poor soil, will produce, not 

 the modified, but the wild form. But Darwin especially 

 points out that it is not these sudden variations which be- 

 come permanent, but those slowly produced by what he 

 terms the accumulative action of changed conditions of life. 



